Doctoral dissertation
To be presented by permission of the Faculty of Medicine of the University of Kuopio for public examination in Auditorium in the Central Finland Central Hospital, Jyväskylä, on Saturday 28th June 2008, at 12 noon
Departments of Medicine and General Medicine Central Finland Central Hospital and University of Kuopio
JUHA SALTEVO
Low-Grade Inflammation and Adiponectin in the Metabolic Syndrome
JOKA KUOPIO 2008
FI-70211 KUOPIO FINLAND
Tel. +358 17 163 430 Fax +358 17 163 410
www.uku.fi/kirjasto/julkaisutoiminta/julkmyyn.html
Series Editors: Professor Esko Alhava, M.D., Ph.D.
Institute of Clinical Medicine, Department of Surgery Professor Raimo Sulkava, M.D., Ph.D.
School of Public Health and Clinical Nutrition Professor Markku Tammi, M.D., Ph.D.
Institute of Biomedicine, Department of Anatomy
Author´s address: Department of Medicine
Central Finland Central Hospital FI-40260 JYVÄSKYLÄ
FINLAND
Supervisors: Academy Professor Markku Laakso, M.D., Ph.D.
Department of Medicine
University of Kuopio
Professor Mauno Vanhala, M.D., Ph.D.
Unit of Family Practice
Central Finland Central Hospital, Jyväskylä Kuopio University Hospital, Kuopio
Reviewers: Professor Johan Eriksson, M.D., Ph.D.
Department of General Practice and Primary Health Care University of Helsinki
Docent Jorma Lahtela, M.D., Ph.D.
Department of Medicine University of Tampere
Opponent: Professor Pirjo Nuutila, M.D., Ph.D.
Department of Medicine University of Turku
ISBN 978-951-27-0955-7 ISBN 978-951-27-1052-2 (PDF) ISSN 1235-0303
Kopijyvä Kuopio 2008 Finland
University Publications D. Medical Sciences 435. 2008. 109 p.
ISBN 978-951-27-0955-7 ISBN 978-951-27-1052-2 (PDF) ISSN 1235-0303
ABSTRACT
The metabolic syndrome (MetS) is an established risk factor cluster for cardiovascular disease (CVD) and type 2 diabetes (T2DM). A rapid increase in the prevalence of obesity worldwide is also associated with an increase of the MetS. Abdominal obesity and insulin resistance are probably the key elements of the syndrome. The role of low-grade inflammation and hypoadiponectinemia remains unclear in the MetS.
The aim of this study was to evaluate the association and significance of low-grade inflammation, measured by high-sensitivity(hs)-CRP and IL-1 receptor antagonist (IL-1 Ra), and adiponectin with the MetS in a population-based study.
The study population consisted of subjects from five different age groups (mean age 46 years) living in the city of Pieksämäki. Altogether 923 of 1 294 (71.3%) individuals participated in the cross-sectional studies in 1997-98. The prevalence of the MetS according to the International Diabetes Federation (IDF) definition was 38% in men and 34% in women. The corresponding figures according to the National Cholesterol Education Program (NCEP) definition were 34%
and 27%.
The levels of pro-inflammatory markers, hs-CRP and IL-1 Ra, were significantly higher among women with the MetS compared to men with the MetS, independently of the definition used. In contrast, no gender difference in these markers between men and women was observed in subjects without the MetS.
The relative change in BMI from childhood to adulthood and the levels of adiponectin and markers of a low-grade inflammation were related. The association was particularly strong among women. Insulin sensitivity correlated significantly with adiponectin and IL-1 Ra levels, independently of confounding factors, but did not correlate with hs-CRP. The levels of adiponectin, hs-CRP and IL-1 Ra were similarly and linearly correlated with the number of components of the MetS in both sexes according to the IDF and NCEP definitions.
In conclusion, the MetS was associated with hypoadiponectinemia and low-grade inflammation measured by hs-CRP and IL-1 Ra in this cross-sectional population-based study.
An association was found between the relative change in BMI between childhood and adulthood, insulin sensitivity, the number of components of the MetS and, above all, female gender. Low-grade inflammation could be one explanation why prediabetic women tend to have a more atherogenic risk profile than males years before the diagnosis of diabetes. This long- lasting inflammatory stress may in part explain why T2DM is related to relatively higher CVD mortality in women than in men.
National Library of Medicine Classification: WB 286, WD 210, WD 200.5.H8, WG 120, WK 810 Medical Subject Headings: Adiponectin; Adult; Body Mass Index; C-Reactive Protein; Cross- Sectional Studies; Diabetes Mellitus, Type 2; Cardiovascular Diseases; Child; Female; Finland;
Humans; Inflammation; Insulin Resistance; Interleukin 1 Receptor Antagonist Protein; Male;
Metabolic Syndrome X; Obesity; Risk Factors
Tiedon määrä on rajaton, mutta sen totuus rajallinen.
"Hiljaa pitää miehen kairassa kulkea ja nostaa hattua kelopuulle"
(Alpiini, Lapin mies)
To my wife Sirke and,
children Ilona, Inari, Saana, Onni and grandchildren Veeti, Aslak and Hilla
This study was carried out in the Departments of Medicine and General Medicine, Central Finland Central Hospital and University of Kuopio. It is based on the metabolic syndrome project conducted in the Pieksämäki area where the first stage screening was done by Professor Mauno Vanhala, M.D., Ph.D., in 1993-94. The subjects were studied again in 1997-98 and this thesis is based on these data.
I wish to express my deepest gratitude to my principal supervisor Academy Professor Markku Laakso, M.D., Ph.D., who in September 2003 telephoned me, when I was visiting Oxford Diabetes Center. He asked whether or not I was willing to do an academic dissertation on the Pieksämäki data. I was happy to answer "Yes" (there was no other choice). All the adiponectin and cytokine measurements were done under his guidance in the scientific laboratory of Kuopio University. His vast experience in writing scientific papers and showing me the way towards shorter and better reports was of the greatest importance during this thesis. I am grateful that he opened up for me a new scientific world alongside the clinical world, which we had studied together at Kuopio University Hospital in 1984-85 under the guidance of Professor (emeritus) Kalevi Pyörälä.
I owe also my deepest gratitude to my second supervisor, Professor Mauno Vanhala, M.D., Ph.D., Unit of Family Practice, Central Finland Central Hospital and University of Kuopio, who is the father of this Pieksämäki metabolic syndrome project. His pioneer works in finding out the prevalence and the best ways to detect the metabolic syndrome on the population level, were the foundation of this study. He has a very long clinical career at health centers, which has taught him straightward thinking. This practical wisdom has helped me in my conversations with him to find the most appropriate, simple and hopefully best ideas about the metabolic syndrome. He trusted, like the father should , throughout these years in me and this thesis.
Thank you, Manu!
During many visits to Äänekoski and with excellent help in statistical problems from biostatistician Hannu Kautiainen from Medcare Foundation I learned much about how statistically valid data is found and turned into figures and tables from the large raw material.
For all friendly visits, relaxed intellectual atmosphere and coffee breaks I thank Hannu and the ladies, Pia and Katja, from Medcare.
I Thank Professor Esko Kumpusalo, M.D., Ph.D., Kuopio University, Department of General Medicine for his intellectual academic and practical conversations of the topics and valuable comments about the articles. I owe a specially warm thank to Professor Sirkka Keinänen- Kiukaanniemi, M.D., Ph.D., from Oulu University, the reliable Lapp Lady with her vast experience in this field for preparing together with excellent statistical help of Jari Jokelainen, M.Sc., the insulin sensitivity paper (Study II).
My warm thanks goes to the head of our Internal Medicine department in the Central Finland Central Hospital, Professor Pekka Hannonen, M.D., Ph.D., who believed in this project and made this work possible. I owe sincere thanks also to Professor Jukka Puolakka, M.D., Ph.D., who is nowadays the doctor-in-chief of our hospital district. 10 years ago Jukka and I published our first paper on the metabolic syndrome in postmenopausal women. During that period Jukka gave me valuable help and tips about scientific work and the problems it involves.
I express my sincere thanks to the official reviewers of this thesis, Professor Johan Eriksson, M.D., Ph.D., University of Helsinki and Docent Jorma Lahtela, M.D., Ph.D., University of Tampere, for their fast and valuable comments and criticism for the improvement of this thesis.
My co-worker, diabetes nurse Marianne Laukkanen, with whom I have done clinical work and conducted many clinical studies during the past twenty years, I warmly thank for her continuous support and excellent clinical work For practical help and psychosocial support I thank Mrs.
Kaija Korpela, M.Sc., and Mrs Nina Peränen, M.Sc., and the D2D project team. I thank also scientific assistant Jani Saalamo, in our hospital district, for his help in making all kinds of figures and posters and Mrs. Aila Ruokokoski, M.A., from the scientific library of our hospital for so quickly finding all the important missing publications and page numbers.
My very best thanks go to all the subjects who participated in this study and especially to study nurses Päivi Lappi and Maarit Kovanen from Pieksämäki Health center who took care of all the study subjects and reliably performed all the measurements.
Finally, I would like to thank those who have supported me during my life. My greatest and warmest thanks goes to my wife Sirke for her endless love and support during our shared life together. She has without doubt had the main responsibility for taking care of our four children Ilona, M.D., Inari, M.Sc., Saana and Onni. We are also proud of our three lovely grandchildren Veeti, Aslak and Hilla. From my father, head of the foniatric department in the Central Finland Central Hospital, Esko Saltevo, M.D., and mother, dentist Pirkko Saltevo, I "inherited" this profession and learned the " never give up" attitude. Thank you for all that. I also warmly thank my friends in the " Poikien kirjakerho", “Vappuporukat” and the cross-country Lapland team with whom I hope to share many more events.
This work was financially supported by Central Finland Health Care District and the department of Internal Medicine of the Central Finland Central Hospital by personal grants for the thesis. Department of General Practice of Kuopio University Hospital gave financial support for the measurements of cytokines.
Jyväskylä, 13 th May 2008
Juha Saltevo
AACE American Association of Clinical Endocrinologists ACE angiotensin-converting enzyme ADA American Diabetes Association
AHA American Heart Association
ANOVA analysis of variance BMI body mass index CB cannabinoid
CHD coronary heart disease CI confidence interval CRP C-reactive protein CVD cardiovascular disease DM diabetes mellitus
DPP Diabetes Prevention Program DPS Diabetes Prevention Study
EASD European Association for the Study of Diabetes ECG electrocardiogram
EGIR European Group for the Study of Insulin Resistance ELISA enzyme-linked immunosorbent assay
ER endoplasmic reticulum FFA free fatty acid
HDL high-density lipoprotein HMW high-molecular weight
HOMA homeostasis model assessment IDF International Diabetes Federation IGT impaired glucose tolerance
IL interleukin
IL-1 Ra interleukin-1 receptor antagonist LDL low-density lipoprotein LMW low-molecular weight MetS metabolic syndrome MI myocardial infarction MMW middle-molecular weight mRNA messenger ribonucleid acid
NCEP National Cholesterol Education Program NEFA non-esterified fatty acid
NGT normal glucose tolerance
NHANES National Health and Nutrition Examination Survey
NO nitric oxide
NS non-significant
OGTT oral glucose tolerance test
OR odds ratio
PAI plasminogen activator inhibitor PCOS polycystic ovary syndrome PVD peripherial vascular disease
QUICKI quantitative insulin sensitivity check index ROS reactive oxyxen species
SD standard deviation
SNP single nucleotide polymorphism TNF tumor necrosis factor
TRL triglyceride rich lipoprotein TZD thiazolidinedione
T2DM type 2 diabetes mellitus
WHR waist-to-hip ratio
This thesis is based on the following publications, which will be referred to their Roman numerals I-IV:
I Saltevo J, Vanhala M, Kautiainen H, Laakso M. Levels of adiponectin, C-reactive protein and interleukin-1 receptor antagonist are associated with the relative change in body mass index between childhood and adulthood. Diabetes and Vascular Disease Research.
2007;4:328-31
II Saltevo J, Laakso M, Jokelainen J, Keinänen-Kiukaanniemi S, Kumpusalo E , Vanhala M. Levels of adiponectin, C-reactive protein and interleukin-1 receptor antagonist are associated with insulin sensitivity: a population-based study. Diabetes/Metabolism Research and Reviews.2008;2Apr (http://www3.interscience.wiley.com/cgi-
bin/fulltext/117952371/main.html,ftx_abs)
III Saltevo J, Vanhala M, Kautiainen H, Kumpusalo E, Laakso M. Association of C- reactive protein, interleukin-1 receptor antagonist and adiponectin with the metabolic syndrome.
The Mediators of Inflammation. 2007; Article ID 93573:1-8
IV Saltevo J, Vanhala M, Kautiainen H, Kumpusalo E, Laakso M. Gender differences in C-reactive protein, interleukin-1 receptor antagonist and adiponectin levels in the metabolic syndrome: a population-based study. Diabetic Medicine 2008;23 Apr:e-pub ahead a print ( DOI.10.1111/j.1464-5491.2008.02440.x)
The original publications are reprinted with a kind permission of the copyright holders.
1. INTRODUCTION...15
2. REVIEW OF LITERATURE...16
2.1. Metabolic syndrome...16
2.1.1. Evolution of the concept of the metabolic syndrome...16
2.1.2. Definitions of the metabolic sydrome...17
2.1.3. Epidemiology of the metabolic syndrome...20
2.1.3.1. Prevalence of the metabolic sydrome...20
2.1.3.2. Metabolic syndrome and the risk of cardiovascular disease and type 2 diabetes ...21
2.1.4. Inflammation and metabolic disorders...23
2.1.5. Future challenges for diagnostics and definition...24
2.2. Etiology and components of the metabolic syndrome...25
2.2.1. Abdominal obesity and fat distribution...26
2.2.1.1. Adipokines secreted by adipose tissue...28
2.2.2. Insulin resistance and glucose intolerance...31
2.2.3. Dyslipidemia...33
2.2.4. Blood pressure...34
2.2.5. Low-grade inflammation and adiponectin in the metabolic syndrome...35
2.2.5.1. Adiponectin...35
2.2.5.2. C-reactive protein...37
2.2.5.3. Interleukin-1 Receptor antagonist...38
2.2.6. Other features in the metabolic syndrome...39
2.2.6.1. Prothrombotic state...39
2.2.6.2. Hyperuricemia...39
2.2.6.3. Endothelial dysfunction...40
2.2.6.4. Microalbuminuria...40
2.2.6.5. Polycystic ovary syndrome...41
2.2.6.6. Depression...41
2.3. Genetics of the metabolic syndrome...41
2.4. Life style and other factors in the metabolic syndrome...43
2.5. Treatment and clinical aspects of the metabolic syndrome...45
2.6. Early life and the metabolic syndrome in adulthood...47
2.7. Gender differences in the metabolic syndrome, diabetes and cardiovascular disease...48
3. AIMS OF THE STUDY...50
4.2. Clinical methods...52
4.3. Assays and calculations...52
4.4. Statistical analyses...53
5. RESULTS...54
5.1. Characteristics of the study subjects...54
5.2. Associations of adiponectin and pro-inflammatory markers with relative change in BMI between childhood and adulthood (Study I)...56
5.3. Associations of adiponectin, C-reactive protein and interleukin-1 receptor antagonist with insulin sensitivity in a population-based cohort (Study II)...59
5.4. The associations of CRP, IL-1 Ra and adiponectin with the metabolic...62
syndrome defined by the NCEP and the IDF criteria (Study III)...62
5.5. Gender differences in CRP , IL-1 Ra and adiponectin levels in the metabolic syndrome defined by the NCEP and the IDF definitions (Study IV)...66
6. DISCUSSION...68
6.1. Study population and design...68
6.2. Study methods...68
6.3. Associations of cytokines and adiponectin with growth between childhood and adulthood...69
6.4. Associations of adiponectin, hs-CRP and IL-1 Ra with insulin sensitivity...70
6.5. Hypoadiponectinemia and pro-inflammation in the metabolic syndrome...72
6.6. Gender differences in hs-CRP, IL-1 Ra and adiponectin levels in the metabolic syndrome...73
6.7. Implications for clinical practice and research...74
7. SUMMARY...75
8. REFERENCES...77
1. INTRODUCTION
The metabolic syndrome (MetS) is the clustering of multiple cardiovascular risk factors in an individual. The constellation of metabolic abnormalities includes glucose intolerance (type 2 diabetes, impaired glucose tolerance, or impaired fasting glycemia), insulin resistance, central obesity, dyslipidemia, and hypertension, all well-documented risk factors for CVD. These conditions co-occur in an individual more often than expected by chance and are associated with increased risk for cardiovascular disease (CVD) (1,2) and type 2 diabetes mellitus (T2DM) (3,4).
Over the past two decades, there has been a striking increase in the number of people with the MetS worldwide. This increase is associated with a global epidemic of obesity and diabetes (5), and the syndrome is seen more often in young adults and children (6).
The pathophysiology of the MetS is complex and not completely understood.The detrimental effects of visceral adipose tissue accumulation and the active endocrine role of adipose tissue have been ackowledged in recent years. Abdominal obesity is believed to be the cause for the MetS, as it clusters with diabetogenic, atherogenic, prothrombotic and proinflammatory metabolic abnormalities (7). Adipose tissue secretes a variety of bioactive substances called adipocytokines, which are closely linked to the MetS and its complications (8). A recently discovered protein, adiponectin, is the most abundant adipocytokine (9), and its expression is highly specific to adipose tissue. Adiponectin has insulin-sensitising properties (10). Reduced adiponectin levels are observed in viscerally obese subjects which contribute to an atherogenic and diabetogenic metabolic risk factor profile (11). In obesity, there is evidence of macrophage infiltration in adipose tissue, leading to an inflammatory condition characterised by elevation of the Interleukin (IL) cytokine superfamily, tumour necrosis factor-Į (TNF- Į) and C-reactive protein (CRP). Adipose tissue produces, presumably as an adaptive response to chronic stress, anti-inflammatory factors suchs as interleukin-1 receptor antagonist (IL-1 Ra) (12). Insulin resistance is increasingly recognized as a chronic, low-grade, inflammatory state.
Atherosclerosis and insulin resistance share similar pathophysiogical mechanisms. This low- grade inflammation could be the link between T2DM and atherosclerosis ("common soil hypothesis") (13).Thus, understanding of the pathophysiology of the MetS and identification of subjects with MetS is highly important. There is urgent need for effective strategies to prevent this emerging global epidemic to be able to prevent the increase of CVD and T2DM (5).
In this population-based study the association and the role of adiponectin, CRP and IL-1 Ra applying two different definitions of the MetS were examined. The association of the above- mentioned proinflammatory markers and adiponectin were also analyzed with relative change in BMI between childhood and adulthood, insulin sensitivity and gender differences in the
prevalence of the MetS in a cohort of 5 age groups born in 1942, 1947, 1952, 1957 and 1962 ( N=1294) in the town of Pieksämäki (population about 20 000), in eastern Finland.
The review of literature will discuss the evolution of the concept, different definitions, epidemiology, and the closely related components of the MetS, especially low-grade inflammation and hypoadiponectinemia. The genetics, lifestyle factors, treatment, early life aspects and gender differences with the MetS will also be discussed.
2. REVIEW OF LITERATURE 2.1. Metabolic syndrome
2.1.1. Evolution of the concept of the metabolic syndrome
Some 250 years ago in the 18 th century Joannes Baptista Morgagni introduced the mechanistic concept in human physiology and pathology. With the help of only knife for anatomical dissection he was able to identify the intra-abdominal and mediastinal fat accumulation in android obesity. He described the association between visceral obesity, hypertension, hyperuricemia, atherosclerosis and obstructive sleep apnea (14). In modern medical literature the same clustering of cardiovascular risk factors, hypertension, hyperglycemia and hyperurikemia, was first described in 1923 by Kylin, a Swedish physician (15). Later, in 1947, the French physician, Vague, drew attention to upper body adiposity (android or male type obesity) as the obesity phenotype that was often associated with metabolic abnormalities, diabetes and cardiovascular disease (16).
In his Banting Lecture professor Gerald Reaven in 1988 described "Syndrome X" (17). This syndrome was "a cluster of risk factors for diabetes and cardiovascular disease" and tightly associated with insulin resistance. Subsequently, the MetS was called " the deadly quartet"(18) or "the insulin resistance syndrome" (19). However, the etiology of the syndrome has remained unclear. In 1998, the World Health Organization (WHO) proposed the first internationally accepted criteria for the MetS (20). The term “metabolic syndrome” was preferred over “insulin resistance syndrome”(Figure 1).
Figure 1. The classical features of metabolic syndrome. Insulin resistance and hyperinsulinemia are the core of the syndrome. Central obesity may also have an etiological role. Dysplipidemia, elevated blood pressure and glucose intolerance are included in the criteria and they might result from the underlying insulin resistance.
2.1.2. Definitions of the metabolic sydrome
Despite general recognition of the syndrome, the lack of knowledge of its etiology and uniform diagnostic criteria complicated epidemiological studies of the MetS for many years. The WHO diagnostic criteria was the first attempt to achieve some agreement on the definition, particularly for research purposes (20). Insulin resistance was the primary abnormality of this diagnosis. It had to be coupled with any two other features of the syndrome (central obesity, elevated blood pressure, dyslipidemia and microalbuminuria). The report clearly stated that the definition did not imply causal relationships and that the definition could he modified as new information was gathered.
The WHO criteria have been criticized because of the inclusion of microalbuminuria as a component of the syndrome. Microalbuminuria in non-diabetic individuals is uncommon (21) and its association with other components of the syndrome is not consistent (22,23).
Furthermore, the requirement that insulin resistance should be measured with the hyperinsulinemic euglycemic clamp technique (24) in glucose tolerant subjects made the criteria
Insulin Resistance
Hyperinsulinemia (Central) Obesity
Elevated blood pressure
Glucose Intolerance Dyslipidemia
TG / HDL
impractical for epidemiological studies. For these reasons the European Group for the Study of Insulin Resistance (EGIR) presented a simpler definition, particularly for epidemiological studies (25). Insulin resistance was still the central element of the diagnosis, but it was defined as the presence of fasting hyperinsulinemia (25,26). Other differences compared to the WHO criteria were the cut-off points for hypertension and dyslipidemia (Table 1). Furthermore, instead of the waist-to-hip ratio (WHR), waist circumference was proposed as a measure of abdominal obesity because it showed a stronger correlation with intra-abdominal adipose tissue mass (27).
Microalbuminuria was not included in the EGIR criteria.
The National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) published their diagnostic criteria for the MetS in 2001 (28). This definition was based on fasting triglycerides, high-density lipoprotein (HDL) cholesterol, plasma glucose, waist circumference and blood pressure with equal weight on each component. Insulin resistance was not included in the definition of the syndrome. American Association of Clinical Endocrinologists (AACE) published their slightly different modified NCEP criteria in 2002 to refocus on insulin resistance as the primary cause of metabolic risk factors. Like the EGIR, they used the name insulin resistance syndrome. Major criteria were IGT, elevated triglycerides, reduced HDL cholesterol, elevated blood pressure, and obesity. No specified number of factors qualified for diagnosis, which was left to clinical judgment (29).
Recently, the International Diabetes Federation (IDF) revised the definition of the MetS (30) to be globally more useful. In the IDF criteria abdominal obesity is recognized as an underlying factor for the syndrome. Therefore, a large waist circumference was defined as a compulsatory component for the diagnosis of the MetS. For the first time ethnic-specific cut-points for waist circumference were given. For Caucasian men the cut-off point is 94 cm and for European women 80 cm, based on the sensitivity and specificity of these cut-off points to identify subjects with body mass index (BMI) >25 kg/ m2 or WHR t0.9 for men and t0.85 for women (31). The cut-off points for waist circumference for Asian populations are lower since their risk for T2DM and CVD is apparent at lower levels of adiposity than in Europeans (32). The criteria for dyslipidemia and elevated blood pressure are identical to those of the NCEP definition, but the limit for hyperglycemia has been lowered to t 5.6 mmol/1 according to the new definition of impaired fasting glucose by the American Diabetes Association (ADA) (33).
Table 1. Definitions of the MetS by the WHO, EGIR, NCEP, ACE, IDF, and updated NCEP criteria (modified from 34). WHO EGIR NCEP AACE IDF Updated NCEP Required f-insulin in top 25%; f- glucose 6.1 mmol/L; 2 h glucose 7.8 mmol/L f-insulin in top 25% — High risk of being insulin resistant Waist 94 cm in men or 80 cm in women
— and 2 of: and 2 of: 3 of: and 2 of: and 2 of: 3 of: Fasting glucose 6.1 mmol/L 6.1 mmol/L 6.1 mmol/L 5.6 mmol/L 5.6 mmol/L HDL cholesterol (mmol/L) <0.9 in men or <1.0 in women <1.0 <1.03 men or <1.29 women<1.03 men or <1.29 women<1.03 men or <1.29 women <1.03 men or <1.29 women or or Triglycerides 1.7 mmol/L >2.0 mmol/L 1.7 mmol/L 1.7 mmol/L 1.7 mmol/L 1.7 mmol/L Obesity Waist/hip ratio > 0.90 in men or > 0.85 women;BMI 30 kg/m2
Waist 94 cm men or 80 cm women Waist 102 cm men or 88 cm women
Waist 102 cm in men or 88 cm in women Hypertension (mmHg) 140/90 or drug treatment 140/90 or drug treatment 130/85 or drug treatment 130/85 or drug treatment 130/85 or drug treatment 130/85 or drug treatment Microalbuminuria u-albumin 20 ȝg/min f=fasting, HDL= high-density cholesterol, u= urinary.
2.1.3. Epidemiology of the metabolic syndrome
2.1.3.1. Prevalence of the metabolic sydrome
The prevalence and consequences of the MetS were difficult to estimate before more generally accepted definitions of the syndrome were published. It has become obvious that simultaneously with an increase in obesity there has been an increase in the prevalence of the MetS. Analysis of data on a representative sample of 8814 men and women in the United States (Third National Health and Nutrition Examination Survey from 1988-1999), estimated that the age-adjusted prevalence of the MetS according to the NCEP criteria was 23.6% (35). The prevalence of the MetS increased from 6.7% among participants aged from 20 to 29 years to 43.5% for participants aged from 60 to 69 years. There were also considerable differences in the prevalence of MetS among different ethnic groups (35). The prevalence was highest among Mexican Americans (31.9%) and lowest among Caucasians (21.6%), African Americans (21.6%) and people reporting "another" race or ethnicity (20.3%). Comparison of the WHO and the NCEP definitions showed that the age-adjusted prevalence of the MetS was quite similar, 25.1% and 23.9%, respectively, but 15-20% of the individuals were classified as having the syndrome based on one definition but not on the other, with equal discordance (36).
In a Finnish study of 1 209 middle-aged men (42-60 years) the prevalence of MetS in 1984- 1989 was 14.2%, according to the WHO (insulin resistance estimated as hyperinsulinemia), and 8.8 % according to the NCEP definitions. The prevalence numbers are rather low, but subjects with diabetes and CVD were excluded from this cohort (2). More recent data from Finland show that according to the modified WHO criteria, the prevalence numbers in the 1992 FINRISK survey, conducted as part of the FINMONICA cardiovascular risk factor study, were much higher in men (38.8 %) and in women (22.2 %). The prevalence rates were also analyzed in different categories of glucose tolerance in this cross-sectional, population-based sample of 2 049 individuals. The prevalence of the MetS was 14.4% in men and 10.1% in women with normal glucose tolerance (NGT), 84.8% and 65.4% among those with impaired glucose tolerance (IGT) and 91.1% and 82.7% in subjects with type 2 diabetes, respectively (37). In the same publication, the Finnish Diabetes Prevention Study (DPS) cohort was analyzed and the prevalence of the MetS in 522 participants with IGT was 78.4% for men, and 72.2% for women (37).
The prevalence of the MetS in a random sample of 207 middle-aged subjects in Tampere and 1 148 subjects in Pieksämäki in 1993-94 were 17% in men and 8% in women. In this study the MetS was defined as a clustering of dyslipidemia (hypertriglyceridemia, low HDL cholesterol or both) and insulin resistance (abnormal glucose tolerance, hyperinsulinemia, or both) (38,39).
Recent data on 1 099 healthy Finnish male military conscripts around 19 years of age showed prevalence numbers of the MetS to be 6.8% according to the IDF and 3.5% by the NCEP (40).
A recent study of 9 669 subjects representative of the Greek population reported a prevalence of the MetS 24.5% according to the NCEP criteria, which is very similar to the prevalence in US and Finnish adults (41). According to the new IDF definition, the age-adjusted prevalence of the MetS in the same Greek population was 43.4% and thus much higher. The significantly higher prevalence of the MetS by the IDF definition was attributable to the lower cut-off points for waist circumference and fasting glucose since most subjects with only one or two NCEP diagnostic criteria had the MetS according to the IDF definition (41).
Little is known about the prevalence of the MetS among children and adolescents. Data from the U.S. on adolescents aged 12-17 years in 1999-2004 showed that the prevalence of the MetS using the IDF definition was about 4.5%. It increased with age, was higher among males (6.7%) than females (2.1%), and was highest among Mexican-Ameican adolescents (7.1%) (42). These figures are possibly higher nowadays, because of the rapid rise in adiposity among youth.
2.1.3.2. Metabolic syndrome and the risk of cardiovascular disease and type 2 diabetes
The main reason for identifying subjects with the metabolic syndrome is the fact that these individuals are at a high risk for developing CVD and T2DM (43).
Several prospective population-based studies have investigated the association between metabolic risk factors and CVD. In a large cohort of about 20 000 men and women, followed for an average of 7 years, the all-cause and CVD mortality increased with increasing number of abnormalities associated with the MetS (44).
In the Helsinki policemen study cohort, the insulin resistance factor during the 22-year follow- up had an age-adjusted hazard ratio of 1.28 for CVD risk and 1.64 for stroke risk. In the same study the lipid factor predicted the risk of CVD, but not that of stroke (45). The MetS also predicted coronary heart disease events in elderly (65-74 years) non-diabetic Finnish men followed up for 7 years (hazard ratio 1.33) (46).
In the Botnia study 4 483 subjects aged 35-70 years were evaluated for cardiovascular risk associated with the MetS applying the WHO definition (insulin resistance estimated by HOMA).
The prevalence of CHD and stroke was 3-fold higher in subjects with the syndrome and their mortality markedly increased (12.0%) during the 7-year prospective follow-up compared to subjects without the syndrome (2.2%) (1).
In the Kuopio Ischaemic Heart Disease Risk Factor Study, a population-based, prospective cohort study of 1 209 Finnish men aged 42 to 60 years at baseline (1984-1989) who were initially without CVD, cancer, or diabetes were followed up until December 1998. Those with the
MetS defined applying the NCEP criteria (waist > 102 cm) had 4.2 times and those with the MetS defined by the other NCEP criteria (waist > 94 cm) 2.9 times higher risk to death from CHD than men without the MetS after adjustment for conventional cardiovascular risk factors.
The corresponding figures, according to the WHO definition of the MetS were from 2.9 (waist- hip ratio >0.90 or BMI 30) to 3.3 (waist 94cm) (2).
The predictive value of the WHO and NCEP criteria were compared in the San Antonio Heart Study (2 815 subjects aged 25-64 years, average follow-up time 12.7 years), where the prevalence rate of the MetS in the general population was 25.2% using both definitions. In the general population the NCEP criteria were predictive of all-cause mortality (hazard ratio 1.5) and CVD mortality (hazard ratio 2.5).The WHO criteria only predicted CVD mortality (hazard ratio 1.6) (47).
The Hoorn Study comprised 615 men and 749 women aged 50 to 75 years without diabetes and a history of CVD at baseline in 1989-1990. The prevalence of the MetS ranged from 17- 32% using different definitions (NCEP,WHO, EGIR). All the definitions increased the risk for incident cardiovascular morbidity and mortality in 10 years about 2-fold (48).
In the ARIC study population men and women fulfilling the NCEP definition of the MetS had a 1.5 to 2 times higher risk for developing CHD or stroke after adjustments, but having the MetS did not improve CHD risk prediction beyond the level achieved by the Framingham Risk Score (49). In contrast in the Scandinavian Simvastatin Survival Study(4S) and Air Force/Texas Coronary Aterosclerosis Prevention Study (AFCAPS/TexCAPS) placebo-treated patients with MetS defined by the NCEP criteria showed increased risk for major coronary events irrespective of their Framingham-calculated 10-year-risk score category (50).
In a prospective 4-year follow-up study of 750 coronary patients who underwent coronary angiography, the NCEP-definition of the MetS conferred a significantly higher risk for vascular events than the IDF definition (51). The MetS, as defined by all the main criteria, also predicted incident end-stage peripherial vascular disease (PVD) in a prospective population based study in the elderly, but only when adjusted for diabetes status. Two of the single components of the MetS, elevated fasting plasma glucose and microalbumiuria, predicted PVD (52).
The other major complication of the MetS is T2DM (3,4). Overall, the risk for type 2 diabetes in patients with the MetS is 3-5-fold higher (53). In the Insulin Resistance Atherosclerosis Study the IDF and NCEP definitions were shown to predict type 2 diabetes equally well (54). A close relationship between the MetS and glucose tolerance was demonstrated in the Botnia study, where the prevalence of the MetS was 10% in women and 15% in men with normal glucose tolerance compared to 42 and 62% in subjects with impaired glucose tolerance (IGT) and 78 and 84% in subjects with T2DM (1).The predictive power of the MetS for incident diabetes in a Chinese high-risk population was tested during a 5-year follow-up.The NCEP, WHO, EGIR and
AACE definitions identified men as having a 3,7-4.5-fold and women a 1.6-2.8-fold risk of developing diabetes during the 5-year follow-up (55).
Finally, in the San Antonio Heart Study, the WHO, NCEP and IDF definitions showed a similar ability to predict incident diabetes, which ability was not fully explained by glucose intolerance.
All definitions predicted incident diabetes independently of age, sex, ethnic origin or family history of diabetes (56).
2.1.4. Inflammation and metabolic disorders
The metabolic and immune systems are among the most fundamental requirements for survival.
Many metabolic and immune response pathways or nutrient-pathogen-sensing systems have been evolutionarily conserved throughout species. As a result, immune response and metabolic regulation are highly integrated and the proper function of each is dependent on the other. This interface can be viewed as a central homeostatic mechanism, dysfunction of which can lead to a cluster of chronic metabolic disorders, particularly obesity, type 2 diabetes and cardiovascular disease (57).
The finding in 1993 that tumour necrosis factor-Į (TNF- Į) is overexpressed in the adipose tissue of obese mice provided the first clear link between obesity, diabetes and chronic inflammation (58). With regard to inflammation, the traditional features of this state do not apply to the diseases in question. In the classic literature, inflammation is described as the principal response of the body invoked to deal with injuries, the hallmarks of which include swelling, redness, pain and fever (tumor, rubor, dolor and calor) (59). This, often short-term, adaptive response is a crucial component of tissue repair and involves the integration of many complex signals in distinct cells and organs. However, the long-term consequences of prolonged inflammation are often not beneficial. Although many of the same mediators are involved in obesity and diabetes, few, if any, of the classic features of inflammation have been observed.
Therefore, this new distinct form of injury response is called low-grade or chronic inflammation.This condition is principally triggered by nutrients and metabolic surplus, and engages a similar set of molecules and signalling pathways to those involved in classical inflammation (57).
Both adipose tissue and the liver have an architectual organization in which metabolic cells (adipocytes or hepatocytes) are in close proximity to immune cells (Kupffer cells or macrophages) and both have immediate access to a vast network of blood vessels. This interface might contribute to the emerging importance of these two organs in the initiation and development of metabolic diseases, particularly in the context of obesity and type 2 diabetes.
This chronic inflammation is characterized by abnormal cytokine production, increased acute-
phase reactants and other mediators, and activation of a network of inflammatory signalling pathways (60,61).
2.1.5. Future challenges for diagnostics and definition
Although the diagnosis of the MetS has been established and accepted by international organizations, the reasons for using the diagnosis have recently been questioned (63).
Diagnosis is warranted if a syndrome reflects a unifying pathological process or predicts future adverse event(s) better than the sum of its components. According to its critics, the current data on the MetS does not fulfill either of these criteria, and thus the labeling of subjects with the term MetS shoud be avoided. In their critical joint statement the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) expressed concerns regarding the MetS that the criteria are ambiguous or incomplete and the rationale for thresholds are ill defined. The value of including diabetes in the definition is questionable and insulin resistance as the unifying etiology is uncertain. It is stated that there is no clear basis for excluding other CVD risk factors as smoking and elevated LDL cholesterol, and therefore the CVD risk associated with the syndrome appears to be no greater than the sum of its parts. The treatment of the syndrome is not different from the treatment of each of its components (62).
Furthermore, the separate components of the syndrome may show different risk profiles (49,63,64), suggesting that the different combinations do not carry equal risk. According to a recent review by Richard Kahn, entitled “ The metabolic syndrome (emperor) wears no clothes”, the dichotomous nature of the diagnosis of the MetS is problematic, because the criteria included in the definition are continuous variables with no upper limits given for any of them (65).
The high intercorrelation between the components of the MetS complicates the use of traditional statistical methods and therefore, confirmatory factor analysis, a multivariate correlation method, has been used to investigate the relationship of the variables included in the syndrome (66,67). Factor analysis identifies statistically independent latent factors underlying the associations of the included variables. If an analysis reveals only a single factor this supports the hypothesis that the MetS has a common etiology e.g. insulin resistance. The other case is that several factors represent distinct physiological phenotypes, which suggests a more complex etiology. However, if a variable is associated with more than one factor, this overlap indicates unifying commonalities between physiological domains and provides an insight into the common factor underlying the syndrome (66). Factor analysis has consistently yielded from 2 to 4 factors for the MetS with a separate obesity-hyperinsulinemia factor (46,68-72) that in many cases also included dyslipidemia. Blood pressure has most consistently loaded on a separate factor (68,69,71,72).
Confirmatory factor analysis supports the current clinical definition of the MetS, as well as the existance of a single factor that links all of the core components (73). The same one factor model was found also in adolescents and the model appeared to be consistent across sex and racial/ethnic subgroups (74). Further investigation is needed to determine whether the core factor is genetic or enviromental (73).
2.2. Etiology and components of the metabolic syndrome
The original description of the MetS was based on the assumption that the syndrome is a clinical manifestation of insulin resistance. However, it is unlikely that insulin resistance can explain all the components of the MetS. As more data have been published and many new features have been proposed as being part of the syndrome (low-grade inflammation, hypoadiponectinemia, endothelial dysfunction and prothrombotic state), abdominal obesity has been nominated as a marker of “ dysfunctional adipose tissue”, and is of central importance in the clinical diagnosis of the MetS (7). Also environmental, possibly preventive, intrauterine and early life factors are involved along with genetics in the pathophysiology of the MetS (Figure 2).
Figure 2. Components of the metabolic syndrome. Diet, genes and physical inactivity contribute to the pathophysiology of the MetS.
Proinflammatory Cytokines (Micro-Inflammation)
DIET GENES PHYSICAL ACTIVITY
Central Obesity
Endothelial dysfunction
Elevated blood pressure
Microalbuminuria Glucose
Intolerance Hyperuricemia
Hypoadipo- nectinemia
Dyslipidemia
Prothrombotic state
Type 2 diabetes Cardiovascular disease
2.2.1. Abdominal obesity and fat distribution
The dramatic worldwide increase in the prevalence of obesity, defined as BMI > 30 kg/m², is the most important underlying cause for the increasing prevalence of the MetS and T2DM (5).
Basically, obesity results from an imbalance between energy intake and energy expenditure.
However, this balance is modified by numerous genetic, environmental and psychososial factors (75). In 1956, Vaque reported for the first time that android-type upper body fat deposition is associated with increased risk for chronic diseases, including diabetes, atherosclerosis and gout (76).
In the NHANES study conducted in the U.S between 1989 and 1994 according to the NCEP criteria 30.1% of men had abdominal obesity, while 5 years later it had increased to 36.0%. A similar increase was observed in women during the same period from 45.7% to 51.9% (77). In a recent Finnish population-based study of 45 to 65-year old persons, the prevalence of abdominal obesity was 68.8% in men and 76.4 % in women, according to the IDF criteria ( 94 cm for men and 80 cm for women), and 36.4% and 51.8%, according to the NCEP criteria (102 for men and 88 cm for women) (78).
Randomly chosen primary care physicians in 63 countries recruited consecutive patients aged 18 to 80 years on 2 prespecified half days. Waist circumference and BMI were measured and the presence of CVD and diabetes mellitus recorded. Overall, 24% of 69 409 men and 27% of 98 750 women were obese (BMI > 30 kg/m²). A further 40% and 30 % of men and women, respectively, were overweight. Increased waist circumference (>102 cm for men and > 88 cm for women) was noted in 29% and 48%, CVD in 16% and 13%, and diabetes mellitus in 13%
and 11% of men and women, respectively. A statistically significant graded increase existed in the frequency of CVD and diabetes mellitus for both BMI and waist (79).
While BMI provides an indicator of overall obesity for epidemiological purposes, a more accurate assessment of total body composition can be obtained by bioelectrical impedance (80), underwater weighing (81) or dual energy X-ray absorptiometry (DEXA) (82). Waist circumference is probably the most important anthropometric measure, because the impact of the distribution of body fat is crucial for the metabolic consequences. Waist circumference correlates with abdominal obesity better than waist-to-hip ratio (WHR) (28). The most reliable assessment of abdominal fat distribution can be obtained with computed tomography (83) or magnetic resonance imaging (84).
Adipose tissue is distributed throughout the body as large homogeneous discrete compartments and as small numbers of cells "marbling" or adjacent to other tissues. Most adipose tissue (about 85% of total adipose tissue mass) is located under the skin (subcutaneous fat) in both lean and obese persons (85). In both sexes, older men and women had a significantly greater increase in visceral fat measured by MRI for a given waist
circumference than younger men and women (< 50 years) (86). The term "visceral fat" is commonly used to describe intra-abdominal fat, and includes both intraperitoneal fat (mesenteric and omental fat), which drains directly into portal circulation, and retroperitoneal fat, which drains into the systemic circulation. The relative contribution of intra-abdominal fat mass to total body fat is influenced by sex, age, race/ethnicity, physical activity, and total adiposity (87). At any given level of waist circumference, the prevalence of diabetes is consistently higher in Asians than in Caucasians (88,89).
Since Vaque's work a large number of both cross-sectional and prospective studies have assessed the impact of body fat distribution and confirmed the link of abdominal obesity with insulin resistance (90,91), the metabolic syndrome (92,93), T2DM (94,95), CVD (96,97) and mortality independently of BMI (98). An increase in visceral fat mass, despite similar total fat mass, has been demonstated in subjects with family history of T2DM compared with subjects without. This suggests that increased visceral fat mass may be an early sign of predisposition to the development of insulin resistance and T2DM (99).
In addition to its function as a store of surplus energy, adipose tissue has multiple regulatory functions in metabolism. Changes in biological properties of adipocytes are thought to contribute to the adverse metabolic effects of abdominal fat tissue. In the normal state the balance between adipose tissue, lipolysis and triglyceride synthesis is carefully governed by energy status, various hormones and the autonomic nervous system. This balance is disrupted in abdominal obesity due to an increase in both fasting and postprandial free fatty acid (FFA) levels (100,101). Furthermore, visceral adipose tissue has been shown to be more sensitive to ȕ-adrenergic lipolysis than subcutaneous adipose tissue because of the larger numbers of ȕ- adrenergic receptors on the cell surface (102). The antilipolytic effect of insulin has shown to be impaired in visceral adipose tissue, resulting in an excess supply of FFAs and causing multiple adverse events closely associated with insulin resistance (103,104). Viscerally obese subjects are also characterized by an exaggerated postprandial triglyceride response (105,106).
However, it is not known whether the storage of an absolute or relative excess amount of triglycerides in abdominal fat depots is directly responsible for increased disease risk or whether such deposition is simply associated with other processes that cause risk (87). In one of the earliest hypotheses, Björntorp proposed that intra-abdominal fat mass was the result of activation of the hypothalamic-hypopituitary-adrenal axis by environmental stress (107-109).
More recently, it has been suggested that the limited ability of subcutaneous fat depots to store excess energy results in "overflow" of chemical energy to intra-abdominal fat tissue and"ectopic"
sites, causing insulin resistance in the liver (110) and skeletal muscle (111), whereas fat accumulation in the liver rather than in skeletal muscle is associated more with features of the MetS (112). Liver fat content has been found to be 4-fold higher in subjects with the MetS than without, independently of age, gender, or body mass index (113), and type 2 diabetic patients
have 80% more liver fat, underestimated by ALT level, than age-, weight-, and gender-matched non-diabetic subjects (114). Even in obese adolescents high visceral and low abdominal subcutaneous fat stores, measured in many different ways are the main determinants of an adverse metabolic phenotype (115).
The biological activity of an adipocyte changes as its lipid storage increases. Compared with small adipocytes, large cells are more insulin-resistant and lipolytic, release more inflammatory cytokines and less adiponectin (116,117), and are more frequently found in people with obesity- related metabolic disorders (118).
These hypotheses are not mutually exclusive, and it is possible that all, including genetic factors, are involved in the association between abdominal fat mass and adverse metabolic consequences.
2.2.1.1. Adipokines secreted by adipose tissue
Adipose tissue is not only involved in the storage and mobilization of lipids but is also an important endocrine organ releasing numerous polypeptides, collectively termed adipokines.The term adipokine is generally applied to biologically active substances found in the adipocytes of adipose tissue (119). Adipokines include a variety of proinflammatory peptides: TNF-Į, Interleukin(IL)-6, CRP, adiponectin, leptin, resistin, visfatin, PAI-1, apelin, hepcidin, vaspin and IL-1 Ra (120) (Table 2).
TNF-Į is a paracrine mediator in adipocytes and appears to act locally to reduce insulin sensitivity of adipocytes (121). This leads to high release of FFA:s and atherogenic dyslipidemia (122). TNF-Į also increases the secretion of other inflammatory mediators (121).
IL-6 is a systemic adipokine, which not only impairs insulin sensitivity, but is also a major determinant of the hepatic production of C-reactive protein (CRP), the most important source of this inflammatory marker (123). The role of CRP in the MetS is discussed later in section 2.2.5.2. During the last decade, an accumulating amount of data has suggested that IL-6 plays a pivotal role as a multifaceted pleiotropic cytokine. IL-6 produced in the working muscle during physical activity could act as an energy sensor by activating AMP-activated kinase and enhancing glucose disposal, lipolysis and fat oxidation (124).
Adiponectin is the most abundant adipokine in the plasma (125). In contrast to other adipokines, adiponectin level is inversely associated with insulin resistance, obesity, the MetS and type 2 diabetes (8,126), and women have higher levels of adiponectin compared to men (127). The role of adiponectin is discussed later in section 2.2.5.1.
Leptin was first identified in 1994 as a hormone secreted by adipocytes. It is a 16 kDa non- glycosylated peptide hormone encoded by the gene obes(ob), the murine homolog of the human gene LEP (128). Adipose tissue is the only known source of leptin, and its secretion is proportional to adipocyte size (129). Leptin participates in the regulation of appetite and energy
expenditure (130), acts as a potential modulator of the hypothalamic-pituitary-adrenal-axis (131), and is considered a hormone with pleiotropic actions (132). Leptin acts as a fundamental signal for the brain to modulate food intake as a function of energy status. Loss of leptin function results in obesity (133). In men (134) and patients with type 2 diabetes (135), plasma leptin levels are associated with the occurrence of cardiovascular diseases. Because the level of leptin increases with obesity and that of adiponectin decreases with obesity, the ratio of leptin to adiponectin can be used as an index of insulin resistance (136). In a recent study with young Finnish adults it was found that the ratio between hs-CRP and leptin was independent of obesity and cardiovascular risk factors. Because the effect of leptin was not restricted to obesity, it was suggested that leptin might regulate development of chronic low-grade inflammation at all levels of body weight (137). In a prospective population-based study leptin levels significantly predicted the development of the MetS, independently of baseline BMI (138).
Resistin is a dimeric protein that is highly expressed in mouse adipose tissue (139). In humans resistin is mainly produced by macrophages and is involved in an inflammatory process that may be related to atherosclerosis (140). Abdominal fat depots showed a 418% increase in resistin mRNA expression compared with thigh fat (141).
Visfatin, originally identified as a pre-B-cell colony-enhancing factor, is highly expressed in visceral adipose tissue, and plasma visfatin levels correlated with obesity (142). Visfatin lowers blood glucose levels in mice, and in vitro visfatin directly activates the insulin-receptor signalling cascade (142). However, in humans no correlation was found between plasma visfatin levels and parameters of insulin sensitivity (143).
High levels of plasminogen activator inhibitor (PAI)-1 contribute to the procoagulant state in the MetS. Although PAI-1 is synthesized in many cell types, adipose tissue is the major source of PAI-1, and circulating PAI-1 levels correlate with visceral obesity (144). In obese individuals PAI-1 levels are increased in those with MetS (145). The fibrinolytic system could play a role in the regulation of adiopose tissue development and insulin signaling in adipocytes (146).
Apelin is a bioactive peptide that was originally identified as the endogenous ligand of orphan G-protein-coupled receptor APJ (147). It has been suggested that production of apelin in the obese might be an adaptive response to obesity-related disorders such as mild chronic inflammation (148).
The most recently discovered adipokines are hepcidin and vaspin. Hepcidin was discovered in 2001 as a urinary antimicrobial peptide synthesized in the liver, and was later identified as an adipokine. It has a function as a key regulator of iron homeostasis, hypoxia and inflammatory stimuli (149,150). Vaspin was discovered in 2005 as a serpin (serine protease inhibitor) produced in visceral adipose tissue. It might also constitute a compensatory mechanism in response to obesity and inflammation (151).Vaspin concentrations are higher in female
compared to male subjects. In the normal glucose-tolerant (NGT) group, circulating vaspin significantly correlated with BMI and insulin sensitivity (152).
Table 2. Overview of key adipokines (modified from 120, 154).
Adipokine Key Properties Secretion in obesity
Tumour Necrosis- Pro-atherogenic, diabetogenic, Increased Factor (TNF)- Į paracrine role in adipocyte
Decreased insulin signalling
Increased secretion of other inflammatory mediators
Interleukin (IL)-6 Promotes inflammation, pro-atherogenic, Increased diabetogenic
Increased vascular inflammation
Increased hepatic C-reactive protein production Decreased insulin signalling
Possible enhances glucose disposal and lipolysis
C-reactive protein Promotes inflammation, pro-atherogenic, Increased diabetogenic
Marker of chronic low-grade inflammation Predicts adverse cardiovascular outcomes
Adiponectin Anti-atherogenic, anti-diabetogenic Decreased Decreased differentation of
macrophages into foam cells
Decreased atherogenic vascular remodelling Decreased hepatic glucose output
Increased insulin sensitivity
Leptin Inhibits appetite and weight gain in hypothalamus Increased Decreased insulin signalling
Pleiotropic actions
Resistin Exarberates insulin resistance Increased Decreased insulin signalling
Decreased endothelial function
Increased vascular smooth muscle proliferation
Visfatin Activates insulin signalling in mice Increased Plasminogen Activator- Pro-atherogenic, pro-coagulant Increased Inhibitor (PAI)-1 Increased atherothrombotic risk
Apelin Adaptive response to obesity (?) Increased Increased by TNF
Hepcidin Regulator of iron homeostasis and inflammatory Increased stimulus
Vaspin Compensatory mechanism Increased to obesity and inflammation (?)
Interleukin-1 Receptor Adaptive response to inflammation Increased antagonist (IL-1 Ra) Prevents IL-1 responses
Acute phase reactant
Increases insulin resistance
Adipose tissue produces, presumably as an adaptive response, anti-inflammatory factors such as interleukin-1 receptor antagonist (IL-1 Ra), which binds competitively to the type 1
receptor without triggering activity within the cell (153). The role of IL-1 Ra is discussed later in section 2.2.5.3.
Accumulating evidence suggests that an increase in visceral abdominal fat mass is the primary perturbation in the pathogenesis of the MetS, providing mediators, adipokines, for cross-talk with other tissues and finally leading to insulin resistance and vascular disorders (154).
2.2.2. Insulin resistance and glucose intolerance
Insulin is a hormone with important metabolic functions. By stimulating the uptake of glucose into skeletal muscle cells and adipocytes and inhibiting hepatic glucose production, insulin can be regarded as the most important regulator of the plasma glucose level. In addition, insulin stimulates lipogenesis and glycogen and protein synthesis in adipose tissue, liver and skeletal muscle cells and inhibits glycogenolysis, lipolysis and protein breakdown (155).
Insulin resistance can be defined as an insufficient response of the target organs (liver, skeletal muscle cells and adipose tissue) to physiological plasma insulin levels. A disruption in the insulin signalling transduction pathway is considered to be the most important underlying mechanism. Insulin resistance has different effects in different organs and leads to a compensatory increase in the production of insulin by pancreatic beta-cells, resulting in hyperinsulinemia (155). The impact of genetic factors has been demonstated by reduced insulin sensitivity in certain ethnic groups (156) and in first-degree relatives of T2DM patients (157).
Also free fatty acids and TNF-Į and other adipokines play a central role. Insulin resistance is a strong predictor of the risk for developing T2DM (158). In large prospective population studies hyperinsulinemia (a surrogate marker of insulin resistance) has also predicted CVD and mortality (159-161).
The hyperinsulinemic euglycemic clamp is widely recognized as the gold standard for measuring insulin sensitivity (24). However, it is complex, costly and time consuming, and therefore not suitable for population-based studies. The homeostasis model assessment (HOMA-IR) of insulin resistance is a simple and less expensive method and therefore widely used in epidemiological studies. HOMA-IR gives an estimate of basal insulin resistance from the mathematical modeling of fasting plasma glucose and insulin concentrations. The correlation with clamp-measured insulin sensitivity is generally around -0.80 (162). Fasting insulin level has also been used as a marker of insulin resistance in epidemiological studies. The correlation of insulin levels and clamp measured insulin sensitivity is around -0.70 in subjects with normal glucose tolerance (26). In the large DECODE study, hyperinsulinemia, defined by the highest quartile for fasting insulin, was significantly associated with cardiovascular mortality in both men and women independently of other risk factors (163). Hyperinsulinemia and insulin
resistance may partly represent different phenotypes (164). The quantitative insulin sensitivity check index (QUICKI) is an alternative method of measuring insulin sensitivity in large population studies, and is determined by fasting glucose and insulin levels. QUICKI correlates significantly with the glucose clamp method (165,166), and predicts better than fasting insulin the onset of type 2 diabetes (167).
The biological action of insulin is mediated via the insulin receptor, which is widely expressed in human tissues. Insulin resistance is believed to result from a defect inherent in the insulin signalling pathway, which is common to many insulin-responsive cell types. Individuals who lack the insulin receptor or have antibodies against it develop severe insulin resistance (168).
However, this intrinsic receptor defect does not explain the link between obesity and insulin resistance. As insulin is anabolic and enhances fat storage, insulin resistance in adipose tissue might be expected to mitigate obesity (169).
An alternative theory postulates that insulin resistance arises when pathological levels of humoral factors disrupt insulin signalling in responsive tissues. This humoral theory emerged from the recently recognized role of adipose tissue as a secretory organ (170), and provided an obvious link between obesity and insulin resistance. In 2005 this was confirmed and extended by demonstrating that inflammation in the liver can be the primary source of systemic factors that lead to the development of insulin resistance (171). Insulin has been found to be an anti- inflammatory hormone. It has been shown to suppress several proinflammatory transcription factors, such as nuclear factor-țB, EGR-1 and activating protein-1 and the corresponding genes regulated by them, which mediate inflammation (172,173). Impairment in the action of insulin because of insulin resistance would thus result in the activation of these proinflammatory transcription factors and an increase in the expression of the corresponding genes (174).
The integration of all these mechanisms might lead to so called endoplasmic reticulum (ER) stress. This theory was presented by Hotamisligil (175). It is believed that inflammatory responses in obesity can be triggered in the adipocyte or macrophages by extracellular mediators such as cytokines or lipids, or initiated through ER stress. Signals from all these mediators converge on inflammatory signalling pathways. The mediators include kinases: c-Jun NH2-terminal kinase and protein kinase C (PKC), and reactive oxygen species (ROS). Once activated they lead to the production of additional inflammatory mediators through transcriptional regulation and this starts a vicious circle. They also directly inhibit insulin receptor signalling (175).
Glucose intolerance develops when insulin secretion from pancreatic ȕ cells fails to compensate for insulin resistance in target tissues. Insufficient insulin action in the liver leads to increased gluconeogenesis and to glycogenolysis of stored glycogen. An increase in FFA levels, due to adipose tissue insulin resistance, contributes to glucose production by stimulating gluconeogenesis (176). Rising hyperglycemia per se also contributes via so-called glucose
toxicity to impaired insulin secretion (177). Diminished insulin secretion patterns, which might be genetically determined, are also observed in first-degree relatives of T2DM even during euglycemia (178,179).
In many subjects, atherosclerosis precedes the development of type 2 diabetes, and cardiovascular complications are often present in newly diagnosed type 2 diabetes patients. In the Euro Heart Survey 31% of patients with acute or chronic CHD had diabetes. In the patients with acute CHD, 36% had IGT, and 22% had newly detected diabetes. In the stable group these proportions were 37% and 14%. Only about 30% of the CHD patients had normal glucose tolerance (180). On the other hand diabetes without prior myocardial infarction was CHD equivalent in an 18-year follow-up study of Finnish subjects, if prior CHD was defined as myocardial infarction. When less stringent criteria for prior CHD were used (myocardial infarction or ischemic ECG changes or angina pectoris), type 2 diabetes carried a larger risk than prior CHD, especially in women (181). The metabolic syndrome and diabetes carry increased risk of stroke (182). These findings indicate that diabetes and cardiovascular disease may share an underlying cause (13,183). It is possible that this common link between T2DM and CVD is insulin resistance (184).
2.2.3. Dyslipidemia
Dyslipidemia in the MetS is characterized by elevated triglycerides (TG) and low levels of HDL- C (20,25,29,30). Plasma LDL cholesterol (LDL-C) levels are often normal in the MetS. A common finding, however, is that LDL particles are smaller and denser than normal (185), a state believed to be associated with increased cardiovascular risk.
The predominant mechanism in the insulin resistance setting is increased influx of FFAs into the liver, which increases hepatic production of triglyceride-rich VLDL particles (186). Insulin resistance may also lead to impaired LPL activity, and thus reduced triglyceride clearence may contribute to elevated triglyceride levels in the MetS (187). A recent study links postprandial elevation of triglyceride-rich lipoproteins (TRL) to high hepatic fat content and low adiponectin levels. Elevation of postprandial TRL exposes the liver to excess FFA and can influence hepatocellular lipid metabolism, leading to hepatic steatosis and disturbance in the insulin signaling pathways (188). Hypertriglyceridemia is an excellent reflection of the insulin resistant condition and one important criteria for diagnosis of the MetS.
In the presence of hypertriglyceridemia, a decrease in the cholesterol content of HDL results from decreases in the cholesterol ester content of the lipoprotein core along with variable increases in triglycerides making the particle small and dense. This process is in part a function of the cholesteryl ester transfer protein (189). This change in lipoprotein composition also results in increased clearance of HDL from the circulation (190). The relation of these changes
in HDL to insulin resistance are probably indirect, arising in concert with the changes in triglyceride-rich lipoprotein metabolism (53).
In addition to HDL, the composition of LDL is modified. With fasting serum triglycerides > 2,0 mmol/l, almost all individuals have a predominance of small dense LDL (191), which might be more atherogenic than buoyant LDL. The propensity towards small dense LDL is attributable to its facilitated transit through the endothelium and tight adherence to intimal proteoglycans, in addition to its susceptibilty to oxidation (192). All these changes are not independent, but related to concominant changes in other lipoproteins and risk factors (193).
2.2.4. Blood pressure
The association of elevated blood pressure and insulin resistance or hyperinsulinemia was first reported more than 40 years ago (194). Insulin resistance is found to be directly correlated with the severity of hypertension (195). About a half of hypertensive subjects are insulin-resistant (196,197). The association of hyperinsulinemia and hypertension has also been confirmed in prospective studies. In the San Antonio Heart Study (198), and in Swedish middle-aged men (199) and women (200), fasting insulin levels were associated with hypertension 8-12 years later.
The underlying mechanisms connecting insulin resistance and hypertension are not clear.
Insulin is a vasodilator when given intravenously to normal-weight subjects (201), with secondary effects on natrium re-absorption in the kidney (202). In insulin-resistant subjects the vasodilatory effect of insulin is lost, possibly impairing endothelium-derived nitric oxide (NO) production (201), but the renal effect of natrium reabsorption is preserved (202).
Hyperinsulinemia also increases the activity of the sympathetic nervous system (203) and modifies transmembrane cation transport (204). Furthermore, locally enhanced glucocorticoid activity in the adipose and muscle tissue of obese subjects contributes to metabolic abnormalities associated with the MetS, and this may lead to hypertension via the activation of the renin-angiotensin system (205). The role of adipokines and cytokines in hypertension and the MetS have not yet been established (206), but adiponectin concentrations have been found to be low in subjects with essential hypertension (207).
Insulin resistance and hyperinsulinemia may in the long term promote increases in blood pressure, complementing many other mechanisms in the pathogenesis of hypertension, or they might even be the primary defect (208).
